1. Field of the Invention
The present invention relates to a process for the approximate calculation of the exponential mean of a string of numbers coded in the floating-point format used to convey certain bit rate information in packet-switched data transmission networks known by the abbreviation ATM standing for "asynchronous transfer mode".
2. Discussion of the Invention
ATM networks provide several categories of data transmission service, one of which is known by the abbreviation ABR standing for "available bit rate". The latter service allows the user to use the passband available over the path of a previously established connection without obliging him/her to make a static reservation of resources when establishing the connection. In return for this flexibility, the two ends of the connection must adapt their bit rate to the passband available. For this purpose, particular ATM cells, known as RM cells, where RM stands for "resource management", make it possible to inform the ends of a connection about the availability of the resources in the network. These cells, generated and processed in real time by the end equipment attached to the network, and also by the ATM switches which form the nodes of the network, comprise fields intended to convey bit rate information. These fields represent for example the current permitted bit rate of the source--which is known by the abbreviation CCR "current cell rate"--, or the bit rate recommended to the source by the network--which is known by the abbreviation ER "explicit cell rate". These fields form the subject of a particular floating-point coding characterized by a nullity bit, 5 exponent bits and 9 mantissa bits.
Switches which have this capability calculate the ER bit rate and write it to the RM cells. Among all the possible algorithms for performing this calculation, one family of algorithms such as that described in the article by MM. Kai-Yeung Siu and Hong-Yi Treng entitled "Intelligent congestion control for ABR service in ATM networks", published in the journal COMPUTER COMMUNICATIONS REVIEW vol. 24 no. 5, 1.10.1994, implements a calculation of the "exponential mean" of the CCR bit rates conveyed by the RM cells. The "exponential mean" is nothing other than the result of a linear filtering obtained at the output of a discrete-time first-order digital filter. Denoting by u.sub.n the string of values which are applied to the filter and x.sub.n the string of states of the filter, the state equation of the filter can be written: calculation of the "exponential mean" of the CCR bit rates conveyed by the RM cells. The "exponential mean" is nothing other than the result of a linear filtering obtained at the output of a discrete-time first-order digital filter. Denoting by u.sub.n the string of values which are applied to the filter and x.sub.n the string of states of the filter, the state equation of the filter can be written: EQU x.sub.n+1 =a.x.sub.n +(1-a).u.sub.n (1)
where a is a parameter less than 1, close to 1, which defines the horizon of the filter. The string x.sub.n thus represents a sort of more or less long-term instantaneous mean of the input values u.sub.n.
Calculation of the "exponential mean" is an operation which must be done very rapidly, typically during the switching time of an ATM packet (cell). By way of example, for a 155 Mbit/s network highway, the switching time of a cell in a switch linked to this highway is around 2.8 microseconds. Given the coding of the data u.sub.n in floating format and the relative complexity of processing of these two multiplications followed by an addition, it is almost indispensable to call upon a microprocessor endowed with the capacity to perform arithmetic operations in this type of floating-point coding. Now, these processors are not at present very cheap.